You probably mean "Unable to host session"! "Unable" is the opposite of "enable", and means that DW could not get it to work. You wrote it as indicating that it does work. Though it doesn't make sense grammatically in English. Anyway, let's not worry about semantics.
I don't have (much) experience with DW multiplayer, I'm just trying to help here figuring out what might be wrong since I know a bit more than the average person about Windows 7.
Could you describe how your computer is connected with your friend's computer? What sort of network exists between you two? Do you have them in a LAN (like being nearby eachother, connected with just a single cable or 2 cables with a switch inbetween), or are you two seperated by the internet? So, then does each of you have a (NAT-)router in between?
There should be a textfile in the game folder called "adapters.txt". Could you post the contents here?
[EDIT]You are not alone:
http://www.sonalystscombatsims.com/p...de09f2ffb7918c
If you are separated by the internet, with routers on either end, then the hoster probably forgot to enable portforwarding packets on his router to his computer hosting the session. This means that the router allows packets to pass (through a specific port number) from the internet to go to the LAN-IP-adress of the DW gamehost computer. If this is not set then the router simply ignores packets from the 'joiner' on the internet. The router acts also as a sort of firewall. Which port is required to be 'opened' I don't know. But is probably mentioned in the DW manual. HOW exactly this is done depends on the manual of the router (of the person hosting the game). The joiner should specify the internet-IP-adress of the router (as opposed to the LAN-IP-adress) of the person hosting the session when he wants to connect.